1. Field of the Invention
The present invention is in the field of digital video and audio recording and pertains particularly to methods and apparatus for recording live, compressed video and audio streams.
2. Discussion of the State of the Art
In the art of video and audio transmission, video and audio data are most often stored and transmitted in a compressed format to save bandwidth. A typical method of video compression Moving Pictures Experts Group (MPEG) involves storing and transmitting only some of the full data video frames of the video-audio stream. A full video frame may be referred to as a key frame known in MPEG compression format as an intra-coded pictures frame (I-frame). Delta frames are video frames that come before and after a key frame. Delta frames are known as predicted (P) frames and bidirectional (B) frames in the compression format that defines the I-frame. For example, in a scene where a car moves across a stationary background, only the car's movements will be stored in the delta frames. P frames generally follow an i-frame and contain only video data that has changed from the preceding i-frame. P-frames rely on the preceding i-frame for the rest of the video data. B frames contain only data that has changed from a preceding frame or data that is different from a next frame.
In some cases compressed video is streamed and recorded in real time. When recording starts, the recording system begins capturing video data including audio if any is present. A problem occurs in many cases where no key frame (i-frame) has been captured at the beginning of the recording process. As a result, the beginning of the recorded video cannot be rendered correctly because it is missing the full video data supplied by the key frame until the key frame arrives and is recorded. The delay in receiving a key frame may be as much as a few seconds where audio is heard but the video is blank or does not display correctly.
The inventor is aware of some proposed solutions to this problem such as snipping the first part of the video stream before the first key frame arrives. The audio must be snipped as well to preserve synchronization between the video and audio. A protocol has been proposed (IETF RFC 5168) that enables a recording system to request that a key frame be sent first by the device generating the video. A problem with this solution is that the sender of the video must implement the protocol, which is not supported by most of the recording systems.
Therefore, what is clearly needed is a system and method for recording compressed video streamed live that solves the problems mentioned above.